1. Field of the Invention
This invention relates to the production of molten glass, particularly a method of providing a supply of molten glass and to a glass melting furnace for use in the performance of the method, wherein heat is supplied to the glass by passage of alternating electric current therethrough.
2. Description of the Prior Art.
One of the problems encountered in methods of supplying molten glass and in glass melting furnaces therefor is that for a given glass composition and for a given throughput, i.e. tonnage to be withdrawn from the furnace over a given period, the glass in the furnace chamber is ideally maintained at or in the region of a specific temperature and undergoes heating in the chamber for or in the region of a specific residence time. It will be appreciated that for a given throughput the residence time is determined by the size of chamber.
In many instances it is required to supply molten glass at variable rates depending upon the nature of the glass utilisation process for which the glass is required and since it is not practicable to alter the volume of the furnace chamber, the variable demand for the supply of molten glass results in the glass being supplied, at different times, which has undergone different periods of residence in the furnace chamber.
In general the size of the furnace chamber will be determined by the maximum throughput which the furnace is required to handle, to ensure that the glass arriving at the point of use is in a satisfactorily refined condition. Thus, any variation in the rate of throughput is likely to be a reduction causing the glass to have a longer residence time in the furnace chamber than is ideal. When the residence time increases due to the reduced input, various undesirable effects occur.
Firstly, unless the temperature of the glass in the furnace is reduced, there is a reduction in the thickness of the blanket of solid state glass making material (normally termed "batch"). If the batch blanket is converted prematurely to the molten state, the heat insulating effect achieved by the presence of the batch blanket over a part or all of the surface of the chamber is lost. Furthermore, if the temperature of the glass in the furnace chamber is reduced, there is a risk of turbulence developing in the body of molten glass which, if it occurs, will tend to produce mixing of the solid state batch material into the body of melt rather than the slow simulation by melting from the underside of the batch blanket.
In addition, if part or all of the blanket is disrupted, then the ambient temperature in the space above the body of molten glass may rise to a level at which the life or satisfactory operation of the batch charger for spreading the batch onto the surface of the glass is impaired.
Accordingly, for throughputs less than the maximum for which the furnace is designed to handle, the temperature of the body of molten glass in the furnace chamber requires to be reduced.
Withdrawal of molten glass from the body is desirably effected in such a mode as to withdraw glass from the lower part of the molten body over as wide an area thereof as possible, and thus withdrawing means is preferably provided which brings about this result. For example, the withdrawal means may comprise an outlet in the lower part of one of the peripheral walls forming part of the lateral boundary of the moulten body of glass, and the bottom wall of the chamber containing such molten body of glass may afford a system of channels leading to the outlet and providing a withdrawal flow path which ensures withdrawal of glass from the lower part of the body over a wide area.
If the viscosity of the glass is lowered by a reduction of the temperature, the pattern of flow to the outlet is disturbed. In particular there is an increased tendency for portions of glass contained in the molten body and which lie adjacent to (especially above) the withdrawal flow path to become entrained into the withdrawal flow, whereas at the ideal operating temperature this would not be the case. Thus, there is a risk that insufficiently refined glass or glass containing particles of solid state batch material may be drawn into the withdrawal flow path particularly from a region adjacent to the surface of the glass and in the vicinity of the outlet.
The present invention has been developed primarily to meet the problem outlined above in a specific form of glass melting furance as disclosed in U.S. Pat Spec. No. 3,757,020. In the glass melting furnace therein disclosed electrode means provided in the chamber define respective heating zones situated in horizontally spaced regions of the chamber between which exists an intervening zone bounded partly by said heating zones and partly by portions of the upstanding peripheral wall of the chamber, and channel means defining a withdrawal flow path is formed in the bottom wall of the chamber. The channel means provide for withdrawal flow from the lower part of each of the heating zones towards the intervening zone, where the two opposing withdrawal flows meet, and from which a further withdrawal flow extends along the lower part of the intervening zone towards an outlet situated in the lower part of one of the peripheral wall portions which forms a boundary at the intervening zone.
When the furnace is operated to raise the temperature of the molten body of glass in the heating zones by the passage of electric current therethrough, and by conductive transference of heat in the intervening zone, to the ideal operating temperature or a temperature in the region thereof, glass withdrawal is as described from positions distributed over a wide area of the lower part of the body of molten glass and the withdrawn glass is replaced by generally descending glass in the furnace chamber itself replaced by melting from the underside of the batch blanket. The arrangement is especially advantageous in two respects. Firstly, if there is any entrainment of glass in the layers near the surface of the body with the withdrawal flow, the fact that withdrawal flow from the horizontally spaced heating zones takes place in opposed directions tends to produce a stationary pool of glass in the intervening zone, and thereby minimises destructive erosion of the peripheral walls of the furnace chamber especially in the vicinity of the outlet. Secondly, because the glass in the intervening zone is not itself directly traversed by the heating current but is heated by heat transfer from the horizontally spaced heating zones, the temperature thereof can be maintained at a value which is sufficient to provide the required melting from the underside of the batch blanket but which is not so high as to accentuate destructive erosion of the peripheral wall in the vicinity of the outlet due to any slight entrainment flow which may still exist.
If, however, the temperature of the glass in the furnace as a whole is lowered, not only is the carefully controlled pattern of withdrawal flow branches disrupted but the entrainment effect in the intervening zone tends to withdraw glass from the surface downwardly in the vicinity of the outlet.